In the considered technical scope, that is to say the high-speed processing of paper sheets, the closest prior art is represented by a stacking device 1 illustrated in FIGS. 1 to 3. Such stacking device 1 comprises classically a stacking table 2 that is movable in vertical translation between at least one position in which it is empty and located substantially in the same plane as the cutter outfeed conveyor 3 (FIG. 1) and at least one other position in which it is loaded with a stack 4 of sheets and located in the same plane as the sheet stacks 4 evacuation conveyor 5. Consequently, when a stack of sheets 4 is being formed (FIG. 2), the sheets accumulate progressively on the stacking table 2, whose level moves downwards as the sheets accumulate and the stack 4 is formed, up to reach the plane of the evacuation conveyor 5 on which they are then transferred (FIG. 3).
Such a configuration implies that the stacking table 2, which has to bear at least temporarily the whole weight of the sheets stack 4 has to have a more heavy and rigid structure the higher the sheet stacks 4 will be. The vertical movement of such stacking table 2 consequently implies powerful and fast actuators that are not only expansive, but also require regular maintenance.
On the other hand, once the complete sheets stack has been formed, the stacking table 2 must perform, in addition to a vertical downwards movement to reach the plane of the evacuation conveyor, also an upwards movement to reach again that of the cutter outfeed conveyor 3 and start a new sheets stacking cycle. During this period of time, which varies according to the final height of the sheets stack 4, the cutter 6 has imperatively to be stopped, leading to a significant slowdown of the production rates of such a stacking device 1, which is not compatible with the rates of the high-speed printers currently used in most of the document processing units concerned by the invention.
Publication FR 2 480 726 describes a device for stacking cardboard blanks in which the stacking table is fixed and the outfeed conveyor is movable and comprises a first upstream end used as a joint for the conveyor and a second downstream end that communicates with the fixed stacking table and moves upwards as the cardboard blanks are stacked, imposing a variation in the length of the outfeed conveyor. To allow this length variation, the outfeed conveyor is equipped with a tensioning device that has the goal of compensating for the length differences between the ends of said conveyor. On the other hand, the stack forms freely, since the stacking table has no guiding and positioning device for the cardboard blanks. Therefore this technology is limited to low speeds and to stacking of materials such as cardboard, which is by definition rigid and heavier than paper. It is not transposable to the processing of paper sheets, which are by definition thin, flexible and light, nor to a high-speed or even very high-speed processing, since it comprises no device that would allow controlling the individual trajectory of the sheets or obtaining properly formed sheet stacks with perfectly aligned edges. This same type of technology is also described in publications FR 1 402 034, JP 8 217308, JP 48 027453, U.S. Pat. No. 3,419,266 and U.S. Pat. No. 2,660,432. But no existing solution responds to the problem set.
Moreover, if the format of the paper sheets to be processed is too large and does not allow stacking them in stacks, the paper sheet have to be stacked in a stepped or in a fish-scale-like manner, which means that they are placed over each other, overlapping partially, on a stacking table provided with a conveyor moving at low speed. It is thus economically interesting to have the possibility of modifying a sheet stacking machine to switch from a “stack stacking” mode to a “stepped stacking” mode. Yet, in the stacking device according to the prior art represented in FIGS. 1 to 3, this modification requires either to remove the stacking table to replace it with a stacking conveyor positioned at the outlet of the cutter, or to provide a “bypass” system to short-circuit the stacking table and to shift the outlet conveyor to the level of the cutter. Switching to the “stepped stacking” mode consequently requires the addition of complementary mechanical elements and machines including motors, belts, detection cells, etc., which form expansive sets and require significant intervention time.
In the cardboard blanks stacking devices described above, this transformation into the “stepped stacking” mode is not possible, since none of the described devices comprises guiding means that would hold the sheets at the stacking table level.